1. Field of the Invention
The present invention relates generally to digital recording and reproducing apparatuses, and, more particularly, digital recording and reproducing apparatuses for digitally recording on recording media, such as a magnetic tape and an optical disc, a high definition television (referred to as HDTV hereinafter) signal transmitted with a bandwidth being compressed in a MUSE (Multiple Sub-Nyquist Sampling Encoding) system and reproducing thus recorded signal.
2. Description of the Background Art
In recent years. HDTVs have been developed for practical use. Each of R, G and B (red, green and blue) signals outputted from a HDTV camera has a bandwidth of 30 MHz, and therefore a HDTV signal as a whole has a wide bandwidth of 90 MHz. In case of broadcasting such a HDTV signal through one channel of a broadcasting satellite at the band of 12 MHz, a bandwidth of the HDTV signal should be compressed (that is, coded with high efficiency) such that it can be transmitted at 27 MHz which is a bandwidth occupied by one channel.
A bandwidth compressing system developed by NH (Nippon Hoso Kyokai or Japan Broadcasting Corporation) for the above described purpose is the MUSE system. The MUSE system is disclosed in detail in (1) "DEVELOPMENT FOR THE MUSE SYSTEM" by Y. Ninomiya et al. on pp. 18-53, No. 2, Vol. 39 of NHK Technical Research issued February 1987; (2) "MUSE SYSTEM" by Y. Ninomiya on pp. 51-58, No. 8, Vol. 42 of Television Journal issued August, 1988 and (3) U.S. Pat. No. 4,692,801.
An outline of the MUSE system will be described hereinafter. FIG. 1 is a schematic block diagram showing a structure of a MUSE encoder for compressing a bandwidth of a HDTV signal according to the MUSE system and outputting the bandwidth-compressed HDTV signal as a MUSE signal.
Referring to FIG. 1, R, G and B of the HDTV signal each having a bandwidth of 30 MHz are supplied in parallel from, for example, a HDTV camera (not shown), and bands of which are limited by a low pass filter (LPF) 1 having a cut-off frequency of 21-22 MHz. After passing through the LPF1, the R, G and B of the HDTV signal are A/D converted by an A/D converter 2 at a sampling frequency of 48.6 MHz. The HDTV signal converted into a digital signal is subjected to a reverse processing with respect to the gamma characteristic of a camera in a gamma processing circuit 3, which is further applied to a matrix circuit 4.
In the matrix circuit 4, R, G, B of the HDTV signal are converted into a luminance (Y) signal and two color difference (C) signals, then bands of the two C signals are further limited by a LPF 5. As a result, the Y signal of 20 MHz and the two C signals each being of 7 MHz are applied to a TCI (Time Compressed Integration) encoder 6. The TCI encoder 6 puts the two C signals into a line sequence and compresses a time base thereof into 1/4, and thereafter which C signals are further time divisional multiplexed into the Y signal to output one series of the TCI video signals.
Then a sub-sampling processing for compressing a bandwidth of the TCI signal is carried out. Described in more detail, a TCI signal of a still picture portion in a picture frame is subjected to a field off-set sub-sampling processing and a frame off-set sub-sampling processing by a sub-sampling circuit 7 for processing still picture and a TCI signal of a motion picture portion is subjected to a line off-set sub-sampling processing by a sub-sampling circuit 8 for processing motion picture. Such various sub-sampling processings are described in detail in the above described references (1), (2) and (3), which are not closely relevant to the present invention., and no further description will be given here.
A motion detection circuit 9 detects the degree of a motion of picture in a motion picture portion, and a mixer 10 adjusts a mixing ratio of outputs of the still picture processing sub-sampling circuit 7 and the motion picture processing sub-sampling circuit 8 in response to the detected degree of the motion. As a result of the sub-sampling processing, the bandwidth of the TCI signal is compressed to 8.1 MHz.
Thereafter, a gamma processing for a transmission path is performed by a gamma processing circuit 11 for the output of the mixer 10 and an emphasis processing is further performed by an emphasis circuit 12. A control signal (described later), a synchronizing signal and a digital audio signal from an audio encoder 14 (described later) are time divisional multiplexed into the thus processed video signal of 8.1 MHz by multiplex circuit 13. By D/A converting thus obtained signal by a D/A converter 15 and limiting band of the D/A converted signal by a LPF 16, a transmission signal according to the MUSE system (referred to as MUSE signal hereinafter) can be obtained.
The transmission according to the MUSE system is a transmission of analog sampled values and the sampling frequency thereof is 16.2 MHz. The above described MUSE signal transmission system having a bandwidth of 8.1 MHz fulfills a first standard of Nyquist at a transmission rate of 16.2 MHz. Accordingly, on a receiving side of the MUSE signal thus transmitted, the received MUSE signal can be completely re-sampled using a clock which is in synchronization with the train of the sampled values and has a clock rate twice the Nyquist frequency (8.1 MHz), that is, a clock rate of 16.2 MHz.
The received MUSE signal is restored to the original HDTV signal basically through a processing reversal to the above described processing for generating the MUSE signal. FIG. 2 is a schematic block diagram showing a structure of a MUSE decoder for restoring a received MUSE signal to a HDTV signal.
Referring to FIG. 2, first the received MUSE signal has a bandwidth limited by a LPF 21, which is further A/D converted by an A/D converter 22. An output of the A/D converter 22 is subjected to the de-emphasis processing by a de-emphasis circuit 24, which is further subjected to a reverse processing with respect to the gamma characteristic of a transmission path by a gamma processing circuit 26. In addition, the output of the A/D converter 22 is also applied to a separation circuit 23, thereby the audio signal which has been time division multiplexed into the received MUSE signal is extracted to be applied to an audio decoder 25.
Furthermore, signal interpolation processing is performed to an output of the gamma processing circuit 26. More specifically, an intra-field interpolation processing is performed by a motion picture processing circuit 27 for a signal of a motion picture portion in the picture frame and inter-frame and inter-field interpolation processings are performed by a still picture processing circuit 28 for a signal of a still picture portion. These signal interpolation processings are also described in the above described references (1), (2) and (3) and no further description will be given here.
A motion detection circuit 29 detects the degree of motion in the motion picture portion. A mixer 30 adjusts a mixing ratio of outputs of the circuits 27 and 28 in response to the detected degree of motion. The TCI video signal thus restored are supplied to a TCI decoder 31 to be TCI decoded into the original Y signal and the two C signals. The bandwidths of the two C signals are limited by a LPF 32. Then, the Y signal and the C signals are subjected to a reverse matrix processing by a reverse matrix circuit 33, so that the original R, G, and B signals are obtained. Outputs of the reverse matrix circuit 33 is D/A converted by a D/A converter 34 and restored as the HDTV signals of R, G, B.
FIG. 3 is a diagram schematically showing a signal form of such a MUSE signal as described above. As the foregoing, the sampling frequency of the MUSE signal is 16.2 MHz, and the signal form of FIG. 3 is numbered at sampling intervals of 16.2 MHz. More specifically, as shown in FIG. 3, 1H period, that is, one scanning line (referred to as a line hereinafter) comprises 480 sampling points, and 1 frame (two fields) is comprised of 1125 lines. Of the 480 sampling points in one line, 11, 94, 347, 95 and one sampling points are respectively allotted to a horizontal synchronizing signal (HD), a C signal, a Y signal, a control signal and a guard portion for preventing signal interference between the Y and C, respectively. The C signal is included in the signal regions having the line numbers 43-558 and 605-1120 wherein R-Y signal and B-Y signal are line-sequentially multiplexed in the odd-numbered lines and the even-numbered lines respectively. The Y signal is included in the signal regions having the line numbers 47-562 and 609-1124. The control signal is included in the signal regions having the line numbers 559-563 and 1121-1125. FIG. 4 shows the contents of the control signal including sub-sampling phase, motion vector amount and codes for discriminating between an FM modulation and an AM modulation in a modulation system in transmitting MUSE signals. The contents of such a control signal is described in detail in the above described references (1), (2) and (3), and no further description will be given here.
The rest of the regions in the signal form of FIG. 3 correspond to a vertical blanking period, wherein the regions having the line numbers 1 and 2 each includes VIT signals used for equalization of a transmission path and the regions having the line numbers 563 and 1125 each includes clamp level signals used for defining an neutral level of the C signal and for AFC. In addition, the signal regions having the line numbers 3-46 and 565-608 each includes audio data and other additional information. The additional information is non-specified, arbitrary data. The regions having the line numbers 43-46 and 605-608 each includes a part of the C signal region.
Now, a recording and reproducing apparatus for the above described MUSE signal will be described. On the other hand, an analog recording system and a digital recording system can be employed as the recording and reproducing system for the MUSE signal, and the former is used for a video tape recorder (VTR) and an optical disk player for FM modulating and recording a MUSE signal. On the other hand, a digital VTR has developed as utilizing the latter system, by which a digital signal, which can be obtained by A/D converting a MUSE signal at a re-sampling frequency of 16.2 MHz, is recorded on a magnetic tape and then reproduced, which is disclosed in Japanese Patent Laying Open No. 61-238184.
FIG. 5 is a schematic block diagram showing a structure of such a digital VTR. In FIG. 5, a MUSE signal supplied to an input terminal 51 in recording from, for example, the MUSE encoder of FIG. 1 is converted into a digital MUSE signal by a digital recording circuit 52, and thereafter subjected to processing such as time base compression and addition of error correcting codes, and further modulated. The modulated digital MUSE signal is supplied to a rotary magnetic head 57 through a switching circuit 55 and a rotary transformer 56 and recorded on a magnetic tape 58. In reproducing, the digital MUSE signal recorded on the magnetic tape 58 is read out by the rotary magnetic head 57 and applied to a digital reproducing circuit 54 through the rotary transformer 56 and the switching circuit 55. The digital reproducing circuit 54 demodulates the digital signal and subjects the demodulated signal to such processings as time base correction and code error correction, thereby outputting the demodulated digital signal as the MUSE signal. The outputted MUSE signal is restored to the original HDTV signal by, for example, the MUSE decoder of FIG. 2 and supplied to a HDTV monitor TV or the like.
FIG. 6 is a block diagram showing in detail the digital recording circuit 52 shown in FIG. 5. In FIG. 6, the MUSE signal inputted to the input terminal 51 has a bandwidth limited by a Nyquist filter 61 which is the LPF of 8.1 MHz, which is converted into a digital signal by an A/D converter 63 thereafter. The MUSE signal is transmitted in the form of the sampled values as described above and the sampled values should be reproduced correctly. For this purpose, a re-sampling clock of 16.2 MHz in synchronization with the MUSE signal is reproduced by a PLL circuit 62, and A/D conversion is performed based on this clock.
The digital MUSE signal outputted from the A/D converter 63 is separated into a video signal, a control signal and other signal such as an audio signal by a separation circuit 64, and each of which is stored in a memory 66. On this occasion, predetermined processings are performed to the audio signal and the control signal by an audio signal processing circuit 65 and a control signal processing circuit 67, respectively.
Then, the audio signal and the video signal stored in the memory 66 are sequentially read out to a parity generating circuit 68. The parity generating circuit 68 generates a vertical parity and a horizontal parity so as to constitute a correction block shown in FIG. 7 and store the same in the memory 66. One unit of the correction block shown in FIG. 7 comprises an audio signal of n.sub.1 words, a video signal of n.sub.2 words and a horizontal parity of n.sub.3 words with respect to a horizontal direction and an audio or video signal of m.sub.1 words and a vertical parity of m.sub.2 words with respect to a vertical direction. In one example of a digital VTR of a practical use, n.sub.1, n.sub.2, n.sub.3, m.sub.1 and m.sub.2 are set to 2, 120, 5, 86 and 4, respectively.
In addition, k blocks of the correction blocks of FIG. 7 constitute one scan track (referred simply to as a track hereinafter) and L scans of the tracks constitute the data of one field. In the digital VTR of the above described embodiment, one field is comprised of 6 tracks and each track is comprised of 4 correction blocks. Namely, the correction block of FIG. 7 is the data corresponding to 1/24 field.
Then, the audio data, the video data, the control signal and the parity are sequentially read out from the memory 66 and distributed to the respective channels so as to be applied to data frame composing circuits 72a and 72b. A plurality of systems (for example two systems) of circuits are provided in the subsequent stages of the data frame composing circuits because the channel distribution recording system is adopted. In addition, the synchronizing signal and the address signal are supplied from a synchronizing signal generating circuit 69 and an address signal generating circuit 70 respectively to both of the data frame composing circuits 72a and 72b. The data frame composing circuits 72a and 72b compose one unit of data frame including the synchronization data of l.sub.1 words, the address data of l.sub.2 word, the data of (n.sub.1 +n.sub.2 +n.sub.3) words comprising the audio, the video and the parity, in this sequence in accordance with the supplied data as shown in FIG. 8 (A) and output the same. More specifically, the data frame shown in FIG. 8(A) corresponds to one horizontal line of data in the correction block of FIG. 7, and the address data of l.sub.2 words includes the data comprising the number for identifying the corresponding data frame, the numbers of the block and the track including the corresponding data frame and the numbers of the field and the frame (picture). The data frame composing circuits 72a and 72b sequentially feed the above described data frames corresponding to respective lines of the correction block and apply the same to the data composing circuits 73a and 73b.
With respect to the control signal, the data frame composing circuits 72a and 72b, in response to the control signal read out from the memory 66, the synchronizing signal and the address signal from the synchronizing signal generating circuit 69 and the address signal generating circuit 72 respectively, compose one unit of the data frame including the synchronization data of l.sub.1 words, the address data of l.sub.2 words and the control signal of (n.sub.1 +n.sub.2 +n.sub.2) words in this sequence as shown in FIG. 8B and output the same to be applied to the data composing circuits 73a and 73b, respectively.
A preamble signal and a postamble signal are supplied from a preamble/postamble signal generating circuit 71 to the data composing circuits 73a and 73b, respectively. The data composing circuits 73a and 73b compose signals of one track having an arrangement as shown in FIG. 9, based on the various data received from the data frame composing circuits 72a and 72b and the preamble/postamble signal generating circuit 71. The signal of one track shown in FIG. 9 comprises a preamble signal of S.sub.1 frames, a control signal of S.sub.2 frames, data of S.sub.4 frames comprising audio, video and parity, a control signal of S.sub.2 frames and a postamble signal of S.sub.3 frames.
The preamble portion and the postamble portion are provided at the opposite edges of the recording track as shown in FIG. 9 for a margin in switching the head, that is, a leading-in time period for clock reproduction and the absorption of rotary jitter of a cylinder and the like, in the digital VTR for recording using a rotary head. The preamble signal and the postamble signal recorded in these portions are usually signals having a fixed frequency corresponding to the maximum value or one fraction of integral number of the recording frequency of the signal to be digitally recorded.
As shown in FIG. 9, the signals each composed one scan track basis are modulated in digital modulation circuits 74a and 74b, and thereafter amplified by recording amplifying circuits 75a and 75b. Then, the signals of two channels outputted from the recording amplifying circuits 75a and 75b are switched into two systems by the switching circuit 55 of FIG. 5 in accordance with a rotating phase of the rotary head, and fed to the rotary magnetic heads 57 through the rotary transformer 56, and thereafter are further recorded on the magnetic tape 58.
FIG. 10 is a block diagram showing in detail the digital reproduction circuit 54 shown in FIG. 5. Referring to FIGS. 5 and 10, in reproduction, the signal recorded on the magnetic tape 58 is read out by the rotary magnetic heads 57 and applied to reproduction (playback) amplifying circuits 81a and 81b in the digital reproduction circuit 54 through the rotary transformer 56 and the switching circuit 55. The signals amplified by the reproduction amplifying circuits 81 and 81b are waveform shaped by waveform equalizing circuits 82a and 82b so as to compensate the characteristics lost in the magnetic recording and reproducing system and applied to demodulation circuits 83a and 83b and PLL circuits 84a and 84b. The PLL circuits 84a and 84b generate clocks based on the applied reproduced signal, apply the clocks to the demodulation circuits 83a and 83b respectively, so, in response thereto, demodulation circuits 83a and 83b demodulate the outputs of the waveform equalizing circuits 82a and 82b to yield the original data signals.
The demodulated digital signals are applied to the synchronization detecting circuits 85a and 85b to detect the synchronization data. Synchronization separating and serial/parallel converting circuits 86a and 86b separate the data from the demodulated digital signals to carry out serial/parallel conversion based on the detected synchronizing signals and then write the separated data into the memory 87 on a word basis. The data written in the memory 87 is sequentially read out to the error correction circuit (ECC) 88, subjected to correction processing and interpolation processing, and written into the memory 87 again.
The data stored in the memory 87 is read out to a MUSE decoder interface circuit 89 to be restored to the original MUSE signal form, and thereafter outputted from the output terminal 53. The form of the MUSE signal outputted from the output terminal 53, namely, whether it is outputted as an analog signal or a digital signal, or how a video signal and an audio signal are separated is determined in accordance with the specification of the MUSE decoder (for example, the decoder shown in FIG. 2) to be connected in the subsequent stage of the terminal 53.
Meanwhile, in the structure of the MUSE signal shown in FIG. 3, the audio data and other additional information (referred simply to as the audio data hereinafter) included in the signal regions indicated as the line numbers 3-46 and 565-608 before they are multiplexed into a MUSE signal are originally binary signals comprising "0" or "1" to be transmitted in serial at a bit rate of 1.35 Mbps. Since the audio data as the binary signal is transmitted in the above described MUSE signal transmission system having a Nyquist frequency of 8.1 MHz, it is subjected to a processing such as a time base compression by the audio encoder 14 (see FIG. 1), and then time division multiplexed into the vertical blanking portion of the MUSE signal shown in FIG. 3.
FIG. 12 is a block diagram showing a circuit structure for the audio data processing included in the audio encoder 14 and an operational principle thereof is disclosed in detail in Japanese Patent Laying-Open No. 62-172874. Referring to FIG. 12, the audio data having the bit rate of 1.35 Mbps inputted from an input terminal 91 has a time base compressed by a time base compression circuit 92, resulting in a signal having the bit rate of 18.225 Mbps. The binary signal of 18.225 Mbps is converted into a ternary signal comprising "0", "1" or "2" by a binary/ternary converting circuit 93, based on a binary/ternary conversion table shown in FIG. 11 and outputted at the frequency of 12.15 Mbaud. The ternary signal is frequency-converted by a frequency conversion circuit 94 and outputted from the terminal 95 as a signal having a frequency of 16.2 MHz. As a result, a sampling frequency of the audio data becomes the same as that of the MUSE video signal, thereby enabling the transmission in the transmission system of the Nyquist frequency of 8.1 MHz. The audio data is further supplied to a multiplex circuit 13 constituting the MUSE encoder shown in FIG. 1 to be time divisional multiplexed into the MUSE video signal.
As the foregoing, audio data multiplexed into a MUSE video signal is basically a digital signal of a ternary value and different from an Y signal and a C signal which are originally analog signals. The following problems occur when the bit number, that is, a resolution, in quantizing such a MUSE signal in the recording circuit of the digital VTR (FIG. 6) is set on the basis of the resolution of the analog value (for example eight bits) to quantize the audio data which is the ternary signal.
More specifically, the audio data in the form of the ternary signal occupies about 8% of the data capacity of the entire MUSE signal, so that when the audio data is quantized by the same resolution as that of the video signal portion that is to be digitally recorded, instead of one bit, as many as eight bits are required for the quantization of the analog signal. As a result, as the digital recording rate increases, recording wavelength is shortened, which, in turn, increases the error in the recorded data.
In case of FM transmitting a MUSE signal generated by the MUSE encoder of FIG. 1 through, for example, satellite broadcasting, emphasis and instantaneous amplitude compression processings should be performed in the emphasis circuit 12 in the MUSE encoder. In addition, in case of receiving the MUSE signal thus processed and restoring the same to the original HDTV signal in the MUSE decoder of FIG. 2, the de-emphasis and instantaneous amplitude expansion processings should be performed in the de-emphasis circuit 24 in the MUSE decoder.
FIGS. 13A-13D and 13F are graphs and waveform diagrams for explaining these various processings. First, in the MUSE encoder of FIG. 1, the video signal which changes white.fwdarw.black.fwdarw.white as shown by waveform (1) in FIG. 13E is subjected to a processing of an emphasis characteristic shown in FIG. 13 (A) by the emphasis circuit 12, so that the edge portions of the video signal are emphasized as shown in waveform (2) in FIG. 13 (E). The signal is subjected to a processing of instantaneous amplitude compression characteristic of FIG. 13 (B) by the emphasis circuit 12, so that the emphasis of the edge portions is suppressed as shown in waveform (3) in FIG. 13E. The signal thus processed is FM transmitted, and after the signal is received waveform (4) in FIG. 13E, it is subjected to the de-emphasis processing of the characteristic shown in FIG. 13 (C) by the de-emphasis circuit 24 in the MUSE decoder of FIG. 2, resulting in a signal having waveform shown in waveform (5) in FIG. 13E. The signal is subjected to an instantaneous amplitude expansion processing of the characteristic shown in FIG. 13 (D) also by the de-emphasis circuit 24 to be restored to the video signal shown waveform (6) in FIG. 13E. These processings are peculiar to an FM transmission of the MUSE signal and is not performed in case of AM transmission.
In case of the FM transmission of the MUSE signal in this manner, the video signal in the MUSE signal is subjected to the emphasis processing. In recording such a MUSE signal by a conventional digital VTR, in order to obtain a level width from a black level to a white level of the reproduced video signal having the eight bits of resolution (-128 to 127) as shown in waveform (6) in FIG. 13E, the video signal to be recorded waveform (4) in FIG. 13E should be quantized with ten bits of resolution (-512 to 511) to be digitally recorded. Namely, generally in the MUSE system, in order to obtain a reproduced video signal having N (N is a positive integer) bit of resolution, digital recording should be performed with (N+2) numbers of quantization bits. As a result, in the digital recording and reproducing apparatus for a MUSE signal such as a digital VTR, as the recording bit rate increases, a recording wavelength is shortened, which in turn, increases errors in the recorded data.